Tongue-in-cheek advice given to couples during their first year of marriage is to put a quarter in a jar each time you make love. Then, during the second year, take a quarter out each time you make love. At the end of the second year, go to a good restaurant with what’s left.

Blame waning desire on growing accustomed to a partner, decreasing importance placed on sex, work and parenting distractions, and natural consequences of the aging process itself.

But not all couples experience sexual decline over the years; some maintain their feelings of passion for decades.

So what’s the secret of these amorous couples?

Understanding the factors that sustain a healthy sex life could help partners rediscover their passion and keep marriages intact. People who report higher levels of sexual desire have fewer thoughts about abandoning their current relationship and are more satisfied with their partner.

An intriguing study recently explored those factors. Published in Social Psychological and Personality Science, researchers from Canada and England designed a study to test whether individual differences in motivation to meet a partner’s sexual needs – termed sexual communal strength – predict long-term sexual desire in couples.

The authors predicted that people with high sexual communal strength would report engaging in sex to pursue partner satisfaction and that those same people would be buffered against experiencing a drop in desire over time. They also expected that high levels of sexual communal strength would fuel sexual desire even more than other powerful factors, such as gender, relationship duration and satisfaction, age, and children in the household.

To test the sexual communal strength theory, researchers conducted a 21-day study of 44 heterosexual couples. The couples ranged in age from 23 to 60 years old; all were living together and two-thirds were married. The average time in a committed relationship was 11 years, ranging from 3 to 39 years. About half of the couples had children.

Couples began the survey on the same day and were instructed to complete a separate diary in private every night for 21 consecutive days. A follow-up survey was given four months after completion of the daily diary study.

Measures of sexual communal strength included questions such as, “How far would you be willing to go to meet your partner’s sexual needs?” “How high a priority for you is meeting the sexual needs of your partner?” and “How happy do you feel when satisfying your partner’s sexual needs?”

There were also measures of relationship satisfaction (“Our relationship makes me happy”), quality of alternatives (“My needs for intimacy could be easily fulfilled in an alternate relationship”), and sexual desire (“My desire for sex with my partner is strong”).

Each day, participants recorded whether they had sex, and if so, answered questions about sexual goals (“to pursue my own sexual pleasure” or “to please my partner”) and sexual desire (“I felt a great deal of sexual desire for my partner today”).

The study found that people who are motivated to meet their romantic partner’s sexual needs experience sexual benefits themselves. Higher scores of sexual communal health translated to greater daily sexual desire, and that this desire persisted over time compared to people with lower sexual communal strength.

These findings help to explain why people who are highly motivated to care for their partner’s sexual needs experience an elevation in their own feelings of sexual desire.

Perhaps this is nothing more than the Golden Rule when it comes to committed relationships: Treat your partner as you would like to be treated, and you’ve got a good chance of keeping the sexual spark alive.